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Processing Of IPv6 Atomic Fragments

Processing Of IPv6 Atomic Fragments
Posted Jan 19, 2013
Authored by Fernando Gont

The IPv6 specification allows packets to contain a Fragment Header without the packet being actually fragmented into multiple pieces (we refer to these packets as "atomic fragments"). Such packets typically result from hosts that have received an ICMPv6 "Packet Too Big" error message that advertises a "Next-Hop MTU" smaller than 1280 bytes, and are currently processed by some implementations as "fragmented traffic". Thus, by forging ICMPv6 "Packet Too Big" error messages an attacker can cause hosts to employ "atomic fragments", and then launch any fragmentation-based attacks against such traffic. This document discusses the generation of the aforementioned "atomic fragments", the corresponding security implications, and formally updates RFC 2460 and RFC 5722 such that fragmentation-based attack vectors against traffic employing "atomic fragments" are completely eliminated.

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Processing Of IPv6 Atomic Fragments

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IPv6 maintenance Working Group (6man) F. Gont
Internet-Draft Huawei Technologies
Updates: 2460, 5722 (if approved) December 29, 2012
Intended status: Standards Track
Expires: July 2, 2013


Processing of IPv6 "atomic" fragments
draft-ietf-6man-ipv6-atomic-fragments-03

Abstract

The IPv6 specification allows packets to contain a Fragment Header
without the packet being actually fragmented into multiple pieces (we
refer to these packets as "atomic fragments"). Such packets
typically result from hosts that have received an ICMPv6 "Packet Too
Big" error message that advertises a "Next-Hop MTU" smaller than 1280
bytes, and are currently processed by some implementations as
"fragmented traffic". Thus, by forging ICMPv6 "Packet Too Big" error
messages an attacker can cause hosts to employ "atomic fragments",
and then launch any fragmentation-based attacks against such traffic.
This document discusses the generation of the aforementioned "atomic
fragments", the corresponding security implications, and formally
updates RFC 2460 and RFC 5722 such that fragmentation-based attack
vectors against traffic employing "atomic fragments" are completely
eliminated.

Status of this Memo

This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.

Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.

Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."

This Internet-Draft will expire on July 2, 2013.

Copyright Notice

Copyright (c) 2012 IETF Trust and the persons identified as the
document authors. All rights reserved.



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This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.


Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5
3. Generation of IPv6 'atomic fragments' . . . . . . . . . . . . 6
4. Updating RFC 2460 and RFC 5722 . . . . . . . . . . . . . . . . 8
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
6. Security Considerations . . . . . . . . . . . . . . . . . . . 10
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 11
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 12
8.1. Normative References . . . . . . . . . . . . . . . . . . . 12
8.2. Informative References . . . . . . . . . . . . . . . . . . 12
Appendix A. Survey of processing of IPv6 atomic fragments by
different operating systems . . . . . . . . . . . . . 13
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 14

























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1. Introduction

[RFC2460] specifies the IPv6 fragmentation mechanism, which allows
IPv6 packets to be fragmented into smaller pieces such that they fit
in the Path-MTU to the intended destination(s). [RFC2460] allowed
fragments to overlap, thus leading to ambiguity in the result of the
reassembly process, which could be leveraged by attackers to bypass
firewall rules and/or evade Network Intrusion Detection Systems
(NIDS) [RFC5722].

[RFC5722] forbid overlapping fragments, specifying that when
overlapping fragments are detected, all the fragments corresponding
to that packet must be silently discarded.

As specified in Section 5 of [RFC2460], when a host receives an
ICMPv6 "Packet Too Big" message advertising a "Next-Hop MTU" smaller
than 1280 (the minimum IPv6 MTU), it is not required to reduce the
assumed Path-MTU, but must simply include a Fragment Header in all
subsequent packets sent to that destination. The resulting packets
will thus *not* be actually fragmented into several pieces, but just
include a Fragment Header with both the "Fragment Offset" and the "M"
bit set to 0 (we refer to these packets as "atomic fragments").
IPv6/IPv4 translators employ the Fragment Identification information
found in the Fragment Header to select an appropriate Fragment
Identification value for the resulting IPv4 fragments.

While these packets are really "atomic fragments" (they can be
processed by the IPv6 module and handed to the upper-layer protocol
without waiting for any other fragments), many IPv6 implementations
process them as regular fragments. Namely, they try to perform IPv6
fragment reassembly with the "atomic fragment" and any other
fragments already queued with the same set {IPv6 Source Address, IPv6
Destination Address, Fragment Identification}. For example, in the
case of IPv6 implementations that have been updated to support
[RFC5722], if a fragment with the same {IPv6 Source Address, IPv6
Destination Address, Fragment Identification} is already queued for
reassembly at a host when an "atomic fragment" is received with the
same set {IPv6 Source Address, IPv6 Destination Address, Fragment
Identification}, and both fragments overlap, all the fragments will
be silently discarded.

Processing of IPv6 "atomic fragments" as regular fragmented packets
clearly provides an unnecessary vector to perform fragmentation-based
attacks against non-fragmented traffic (i.e., IPv6 datagrams that are
not really split into multiple pieces, but that just include a
Fragment Header).

IPv6 fragmentation attacks have been discussed in great detail in



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[I-D.gont-6man-predictable-fragment-id] and [CPNI-IPv6], and
[RFC5722] describes a specific firewall-circumvention attack that
could be performed by leveraging overlapping fragments. The possible
IPv6 fragmentation-based attacks are, in most cases, "ports" of the
IPv4 fragmentation attacks discussed in [RFC6274].

Section 3 describes the generation of IPv6 "atomic fragments", and
how they can be remotely "triggered" by a remote attacker. Section 4
formally updates [RFC2460] and [RFC5722] such that the aforementioned
attack vector is eliminated. Appendix A contains a survey of the
generation and processing of IPv6 atomic fragments in different
versions of a number of popular IPv6 implementations.







































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2. Terminology

IPv6 atomic fragments
IPv6 packets that contain a Fragment Header with the Fragment
Offset set to 0 and the M bit set to 0.

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].










































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3. Generation of IPv6 'atomic fragments'

Section 5 of [RFC2460] states:

In response to an IPv6 packet that is sent to an IPv4 destination
(i.e., a packet that undergoes translation from IPv6 to IPv4), the
originating IPv6 node may receive an ICMP Packet Too Big message
reporting a Next-Hop MTU less than 1280. In that case, the IPv6
node is not required to reduce the size of subsequent packets to
less than 1280, but must include a Fragment header in those
packets so that the IPv6-to-IPv4 translating router can obtain a
suitable Identification value to use in resulting IPv4 fragments.
Note that this means the payload may have to be reduced to 1232
octets (1280 minus 40 for the IPv6 header and 8 for the Fragment
header), and smaller still if additional extension headers are
used.

This means that any ICMPv6 "Packet Too Big" message advertising a
"Next-Hop MTU" smaller than 1280 could trigger the generation of the
so-called "atomic fragments" (i.e., IPv6 datagrams that include a
Fragment Header, but that are composed of a single fragment, with
both the "Fragment Offset" and the "M" fields of the Fragment Header
set to 0). This can be leveraged to perform a variety of
fragmentation-based attacks [I-D.gont-6man-predictable-fragment-id]
[CPNI-IPv6].

From a security standpoint, this situation is exacerbated by the
following factors:

o Many implementations fail to perform validation checks on the
received ICMPv6 error messages, as recommended in Section 5.2 of
[RFC4443] and [RFC5927]. It should be noted that in some cases,
such as when an ICMPv6 error message has (supposedly) been
elicited by a connection-less transport protocol (or some other
connection-less protocol being encapsulated in IPv6), it may be
virtually impossible to perform validation checks on the received
ICMPv6 error messages.

o Upon receipt of one of the aforementioned ICMPv6 "Packet Too Big"
error messages, the Destinations Cache is usually updated to
reflect that any subsequent packets to such destination should
include a Fragment Header. This means that a single ICMPv6
"Packet Too Big" error message might affect multiple communication
instances (e.g., TCP connections) with such destination.

o Some implementations employ predictable Fragment Identification
values, thus greatly improving the chances of an attacker of
successfully performing fragmentation-based attacks



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[I-D.gont-6man-predictable-fragment-id].


















































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4. Updating RFC 2460 and RFC 5722

Section 4.5 of [RFC2460] and Section 4 of [RFC5722] are updated as
follows:

A host that receives an IPv6 packet which includes a Fragment
Header with the "Fragment Offset" equal to 0 and the "M" bit equal
to 0 MUST process such packet in isolation from any other packets/
fragments, even if such packets/fragments contain the same set
{IPv6 Source Address, IPv6 Destination Address, Fragment
Identification}. A received "atomic fragments" should be
"reassembled" from the contents of that sole fragment.

The Unfragmentable Part of the reassembled packet consists of
all headers up to, but not including, the Fragment header of
the received atomic fragment.

The Next Header field of the last header of the Unfragmentable
Part of the reassembled packet is obtained from the Next Header
field of the Fragment header of the received atomic fragment.

The Payload Length of the reassembled packet is obtained by
substracting the length of the Fragment Header (that is, 8)
from the Payload Length of the received atomic fragment.

Additionally, if any fragments with the same set {IPV6 Source
Address, IPv6 Destination Address, Fragment Identification} are
present in the fragment reassembly queue when the atomic fragment
is received, such fragments MUST NOT be discarded upon receipt of
the "colliding" IPv6 atomic fragment, since IPv6 atomic fragments
MUST NOT interfere with "normal" fragmented traffic.




















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5. IANA Considerations

There are no IANA registries within this document. The RFC-Editor
can remove this section before publication of this document as an
RFC.














































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6. Security Considerations

This document describes how an attacker can exploit ICMPv6 "Packet
Too Big" error messages to cause further IPv6 packets to include a
Fragment Header, such that he can perform any fragmentation-based
attack against otherwise non-fragmented traffic. This document
updates [RFC2460] and [RFC5722], such that the aforementioned attack
vector is completely eliminated.











































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7. Acknowledgements

The author would like to thank (in alphabetical order) Tore Anderson,
Ran Atkinson, Remi Despres, Brian Haberman, Timothy Hartrick, Steinar
Haug, Philip Homburg, Simon Perreault, Florian Weimer, and Bjoern A.
Zeeb, for providing valuable comments on earlier versions of this
document. Additionally, the author would like to thank Alexander
Bluhm, who implemented this specification for OpenBSD.

This document is based on the technical report "Security Assessment
of the Internet Protocol version 6 (IPv6)" [CPNI-IPv6] authored by
Fernando Gont on behalf of the UK Centre for the Protection of
National Infrastructure (CPNI).

Fernando Gont would like to thank CPNI (http://www.cpni.gov.uk) for
their continued support.



































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8. References

8.1. Normative References

[RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", RFC 2460, December 1998.

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.

[RFC4443] Conta, A., Deering, S., and M. Gupta, "Internet Control
Message Protocol (ICMPv6) for the Internet Protocol
Version 6 (IPv6) Specification", RFC 4443, March 2006.

[RFC5722] Krishnan, S., "Handling of Overlapping IPv6 Fragments",
RFC 5722, December 2009.

8.2. Informative References

[RFC5927] Gont, F., "ICMP Attacks against TCP", RFC 5927, July 2010.

[RFC6274] Gont, F., "Security Assessment of the Internet Protocol
Version 4", RFC 6274, July 2011.

[CPNI-IPv6]
Gont, F., "Security Assessment of the Internet Protocol
version 6 (IPv6)", UK Centre for the Protection of
National Infrastructure, (available on request).

[I-D.gont-6man-predictable-fragment-id]
Gont, F., "Security Implications of Predictable Fragment
Identification Values",
draft-gont-6man-predictable-fragment-id-02 (work in
progress), March 2012.

















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Appendix A. Survey of processing of IPv6 atomic fragments by different
operating systems

This section includes a survey of the support of IPv6 atomic
fragments in popular operating systems, as tested in October 30,
2012.

+---------------------+---------------------+-----------------------+
| Operating System | Generates atomic | Implements this |
| | fragments | specification |
+---------------------+---------------------+-----------------------+
| FreeBSD 8.0 | No | No |
+---------------------+---------------------+-----------------------+
| FreeBSD 8.2 | Yes | No |
+---------------------+---------------------+-----------------------+
| FreeBSD 9.0 | Yes | No |
+---------------------+---------------------+-----------------------+
| Linux 3.0.0-15 | Yes | Yes |
+---------------------+---------------------+-----------------------+
| NetBSD 5.1 | No | No |
+---------------------+---------------------+-----------------------+
| NetBSD-current | No | Yes |
+---------------------+---------------------+-----------------------+
| OpenBSD-current | Yes | Yes |
+---------------------+---------------------+-----------------------+
| Solaris 11 | Yes | Yes |
+---------------------+---------------------+-----------------------+
| Windows XP SP2 | Yes | No |
+---------------------+---------------------+-----------------------+
| Windows Vista | Yes | No |
| (Build 6000) | | |
+---------------------+---------------------+-----------------------+
| Windows 7 Home | Yes | No |
| Premium | | |
+---------------------+---------------------+-----------------------+

Table 1: Processing of IPv6 atomic fragments by different OSes

In the table above, "generates atomic fragments" notes whether an
implementation generates atomic fragments in response to receved
ICMPv6 Packet Too Big error messages that advertise a MTU smaller
than 1280 bytes.









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Author's Address

Fernando Gont
Huawei Technologies
Evaristo Carriego 2644
Haedo, Provincia de Buenos Aires 1706
Argentina

Phone: +54 11 4650 8472
Email: fgont@si6networks.com









































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